New Editor-in-Chief Adam Wax introduces himself and a new paper type: the patent review.

Quasi-cyclic low-density parity-check (QC-LDPC) codes with low-complexity hardware implementation and excellent performance have become a hot research topic in many optical communication systems. To meet the different requirements of code lengths in optical communication systems, lifting methods which remove all the short cycles equally are usually used to construct long QC-LDPC codes of larger lengths by lifting short QC-LDPC codes. A lifting method is proposed, which considers both length and connectivity of short cycles. The proposed lifting method reduces the complexity of calculating the approximated cycle extrinsic message degree -related metrics and improves connectivity of the Tanner graph. The numerical results prove that, by employing our proposed lifting method, the lifted long QC-LDPC codes have better bit error rate performance compared with the lifted codes employing the traditional lifting method.

The images acquired in haze conditions are significantly degraded due to the presence of atmospheric particles. These images have low contrast, poor visibility, and some information is lost as well. These characteristics severely hinder the further processing and the applications in which the images are being used. The polarimetric dehazing methods are effective for direct imaging and enhancing the imaging quality. In addition, polarimetric dehazing methods have the capability to cope with haze and other turbid mediums. Therefore, the polarimetric dehazing methods are extensively developed and used in different applications due to their superior performance. We present in detail the principles, the implementation techniques, and the advancements in the polarimetric dehazing methods. We believe that this work is the first in-depth review on the passive polarimetric dehazing methods.

The guest editors summarizes the contributions to Special Section on Laser Damage V.

Picosecond pulse duration laser–material interactions are extremely complex and much less studied than the physics of femtosecond and nanosecond ablation. Additionally, multimode laser beam structure can inhibit robust analysis and comparisons of effects at any pulse duration. To address these gaps, single-pulse laser ablation of Al, Si, Ti, Ge, and InSb in air and Ge in vacuum was studied using low-transverse order Gaussian beams at a 1064 nm wavelength and 28 ps pulse duration. Crater depths of 0.4 to 6.3  μm and volumes of up to 4000  μm³ were measured using a laser confocal microscope. Crater depths plateau with increasing fluence and are slightly higher for Ge in vacuum than in air. Crater volume increases linearly with fluence for all materials in air. In vacuum, the volume of material above the surface was less than in air and increased at a lower rate with increasing fluence. The ratio of volume above the surface (due to melt flow and redeposition) to volume below the surface plateaus for all materials to ∼0.7 in air and 0.4 for Ge in vacuum. The ablation efficiency, defined as atoms removed per incident photon, was higher at low fluences and decreased to ∼0.004 for all materials at higher fluences. Simulations using the directed energy illumination visualization tool showed that bulk melt flow out of the crater caused by the evaporation recoil pressure dominated at higher fluences. Plateauing of crater depth with fluence was caused by melt reflow into the crater, which effects smaller crater widths more than larger ones, as evidenced by comparing multimode results with TEM₀₀ simulations. Recondensation of evaporated material was identified as the main difference between craters formed in air versus vacuum, and the Knudsen layer jump conditions in DEIVI were modified to account for an estimated ≈20  %   recondensation rate. The simulations showed a resulting reduction in evaporation, which created less recoil pressure, driving less melt out of the crater. The results of this study help elucidate mass removal mechanisms in the picosecond pulse duration regime and their dependence on laser and environmental characteristics.

Nanosecond UV laser-induced surface damage of potassium dihydrogen phosphate samples was investigated and discussed by means of defect characterization, in situ damage test, as well as pump–probe shadowgraph imaging. Two distinctive types of surface damage induced by different defects have been demonstrated. Surface damage occurring at relative lower fluence (typically below 5  J  /  cm² in our experiment) is highly correlated with fluorescent surface defects, which are considered as fractural structures introduced by surface cutting. The other type of surface damage that always occurs at higher fluence (above 8  J  /  cm²) is confirmed to originate from the bulk damage precursor located near the crystal surface.

We study laser-induced contamination (LIC) as a potential cause of optical losses and laser-induced damage of optical components for ultrashort pulse lasers with high average power in the MHz regime. Our work is conducted on dichroic mirrors designed for maximum reflection at 515 nm operated in ambient air. Based on the development of an experimental set-up for real-time monitoring of LIC and accelerated test protocols, we have conducted a parametric study on LIC development and studied its growth dynamics and morphology. We show that LIC is a main limitation of short-wavelength, high average power fs/ps lasers, with the formation of nanometric highly absorbing layers of carbonate compounds on the laser footprint, with evidence of thermal effects. It is also found that the last layer of the stack, at the interface between air and coating stack, is critical in the LIC growth which can open some perspectives for limitation of this effect.

The standardization and comparison of laser-damage protocols and results are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured in a round-robin experiment involving five well-equipped damage testing facilities. Investigations were conducted at the wavelength of 1  μm in the sub picosecond pulse duration range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These characteristics enable direct comparison of damage-threshold measurements between different facilities, with the difference in the measured values indicating systematic errors or other parameters that were not previously appreciated. The results of this work illustrate the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, sample homogeneity, and nonlinear beam propagation.

Max-of-N fluence is the maximum peak fluence at a given location over N number of shots and is important for calculating fluence dependence for intrinsic laser-induced optic damage. Previously, we observed the Max-of-N effect on the National Ignition Facility and developed an ad-hoc model to calculate its effect. In this work, we attempt to understand the fundamental mechanism that causes this Max-of-N effect. We conclude that the primary fundamental mechanism responsible for this effect is dominated by the combination of fluence variations and pointing jitter of the laser. This discovery both strengthens our model for predicting optics longevity and gives us insight into how to mitigate this effect.

High-power ArF lasers for high-volume production of mobile consumer microelectronics demand low loss and laser durable calcium fluoride (CaF₂) laser-windows. Increased laser durability is realized by F-SiO₂ protective coatings on CaF₂ windows with subsurface-damage-free (SSD-free) surface finishing. Surface reflection loss is reduced by fluoride-based anti-reflection (AR) coatings. In this contribution, we integrated the protective coating with the fluoride-based AR coatings on SSD-free CaF₂ windows and assessed the laser damage resistance of the window using an ArF laser with a pulse length of 20 ns and a rep rate of 20 Hz. Damage initiation sites were detected on the entrance surface at a fluence of 3.5  J  /  cm². A 100% damage probability was determined at a fluence of 5  J  /  cm². An absorption-driven thermal damage mechanism was indicated by the damage morphology. Similar laser damage resistance was realized on the protected AR-coated windows with more than 9% transmission gain when compared to that of the F-SiO₂ protected CaF₂ laser-windows. The results suggest that two-photon absorption is a dominant factor when nonlinear absorption occurs in the laser-irradiated CaF₂ windows. The thermally induced damage could be further simulated using the experimentally determined two-photon absorption coefficient based on the ArF laser calorimetry technique.

Understanding of the laser-induced damage threshold and impact of air–vacuum cycling of the optical components in short-pulse laser systems is of fundamental importance. We report the results of a damage-testing campaign that monitored representative pulse compression grating samples that were positioned inside the OMEGA EP grating compressor vacuum chamber during normal operation and routinely damage tested on every quarterly vent for a period of about 10 years. The evolution of their damage resistance under 10-ps and 100-ps pulse lengths are associated with a significant degradation of the laser-induced damage thresholds, which is comparatively larger at 100 ps, is described.

The interactions of microparticles of different materials located on the surface of a multilayer dielectric mirror with intense 1053-nm laser pulses of varying fluence and duration (10 and 0.6 ps) are investigated. The particles caused localized intensification of the electric field, which becomes the dominant mechanism for the onset of damage and secondary contamination of the mirror at fluences far below the pristine (without particles) laser-induced-damage threshold of the mirror. Several interaction mechanisms leading to material modification and damage are identified, including the localized field intensification by multibeam interference and particle-induced microlensing, plasma-induced scalding, and secondary contamination via nanoparticle generation and particle melting. The resulting morphologies were observed to be vulnerable to damage growth and additional damage initiation when irradiated by subsequent pulses.

The onset of laser-induced damage on the exit surface of fused silica optics when exposed to UV ns pulses is a limiting factor in the design and operation of most high-energy laser systems. As such, significant effort has been expended in developing laser damage testing protocols and procedures to inform laser system design and operating limits. These tests typically rely on multiple laser exposures for statistical validation. For large aperture systems, testing an area equal to that of the optical components in the system is functionally impossible; instead, sub-scale witness samples are interrogated with elevated fluences. We show that, under certain circumstances, the laser exposure used to test one location on a sample will generate additional, laser-induced damage precursors in regions beyond that exposed to laser light and hence degrade the damage performance observed on subsequent exposures. We hypothesize the precursors result from condensation of vaporized silica, which re-condenses as SiOx particles deposited on the exit surface. In addition, we will outline the conditions under which this phenomenon occurs, as well as methods for mitigating or eliminating the effect.

We investigate the depth of field (DoF) enhancing capacity of binary annular phase masks embedded in panchromatic imaging systems. We first demonstrate with numerical simulations and real-world imaging experiments that phase masks optimized for monochromatic illumination are somewhat robust to their use under wide spectrum illumination: they provide images that are slightly less sharp but less affected by deconvolution artifacts due to spectral averaging. Then, we show that masks specifically optimized for wide spectrum illumination perform better under this type of illumination than monochromatically optimized phase masks under monochromatic illumination, especially when the targeted DoF range is large. This interesting effect comes from the fact that deconvolution artifacts are significantly reduced by wide spectrum illumination. These results show that it is useful to take into account the illumination spectrum together with the scene characteristics and the targeted DoF range for effective co-design of DoF enhancing imaging systems.

This paper proposes a method to estimate the bispectral characteristics of fluorescent objects by using multispectral imaging data. We suppose that an image acquisition system allows multiple illuminant projections to the object surface and multiple response channels in the visible range. The Donaldson matrix, representing the bispectral characteristics, is modeled as a two-dimensional array with the excitation range (350, 700 nm) and the reflection and emission ranges (400, 700 nm). The outputs of a multispectral imaging system are described using the spectral sensitivities of a camera and the spectral functions of reflectance, emission, and excitation. The problem of estimating the spectral functions is formulated as a least-squares problem to minimize the residual error of the observations and the roughness of the spectral functions. We assume that the spectral functions are represented by a limited number of basis functions. We utilize the fact that a fluorescent object usually contains a single fluorescent material (fluorophore). The spectral reflectance and the emission spectrum are estimated using the basis functions, whereas the excitation spectrum is estimated using a physical model between excitation and reflection. An iterative algorithm is developed to obtain the optimal estimates of the three spectral functions of reflectance, emission, and excitation. The performance of the proposed method is examined in simulation experiments and compared with the other methods.

We present a deep learning approach for restoring images degraded by atmospheric optical turbulence. We consider the case of terrestrial imaging over long ranges with a wide field-of-view. This produces an anisoplanatic imaging scenario where turbulence warping and blurring vary spatially across the image. The proposed turbulence mitigation (TM) method assumes that a sequence of short-exposure images is acquired. A block matching (BM) registration algorithm is applied to the observed frames for dewarping, and the resulting images are averaged. A convolutional neural network (CNN) is then employed to perform spatially adaptive restoration. We refer to the proposed TM algorithm as the block matching and CNN (BM-CNN) method. Training the CNN is accomplished using simulated data from a fast turbulence simulation tool capable of producing a large amount of degraded imagery from declared truth images rapidly. Testing is done using independent data simulated with a different well-validated numerical wave-propagation simulator. Our proposed BM-CNN TM method is evaluated in a number of experiments using quantitative metrics. The quantitative analysis is made possible by virtue of having truth imagery from the simulations. A number of restored images are provided for subjective evaluation. We demonstrate that the BM-CNN TM method outperforms the benchmark methods in the scenarios tested.

To improve the quality of the sea surface image under foggy weather, we propose an innovative single-image defogging algorithm based on the dark channel prior principle. The median filter is combined with the minimum filter for the purpose of obtaining accurate dark channel values in areas where the depth of field changes sharply. Then, the algorithm constructs a fog density detection model to obtain the atmospheric light value from the dense fog area and effectively eliminate the interference of bright spots. Subsequently, the atmospheric scattering model is applied to obtain a preliminary fog-free image. Finally, an adaptive logarithmic mapping algorithm is introduced to enhance the visual effect of the defogged image. Experimental results show that the proposed algorithm can effectively improve image quality degradation and avoid halos in the intersection region of sky and sea, Moreover, the method does not require guided filtering for transmittance refinement, which greatly improves the execution speed of the algorithm.

The time-of-flight (TOF) camera has recently received significant attention due to its small size, low cost, and low-power consumption, which can be widely used in fields such as automatic navigation and machine vision. The TOF camera can calculate 3D information of targets with dozens of frames per second. However, poor accuracy still exists in the presence of various inevitable disturbances. In particular, the imaging distance and object reflectivity are remarkable factors. In this study, the depth imaging conditions, including ambient light, detection distance, and object reflectivity, are theoretically analyzed using differential entropy. Because many coupled factors disturb the imaging accuracy simultaneously, we propose a type of supervised learning machine, entropy-based k-nearest neighbor, based on differential entropy. Experiments show that this method can significantly improve the accuracy of depth data obtained by a TOF camera.

A signal-to-noise performance of an imaging Michelson interferometer based on corner cubes is modeled and experimental results are presented. The influence from the background radiation of the uncooled sensor is explained. The noise equivalent spectral radiance is determined in the 700 to 1300  cm^−  1 range at two spectral resolutions using blackbody sources. The performance of the sensor system is tested on a scene at an average temperature of 19°C. From these results, the performance is given in terms of number of samples and spectral resolution. The performance model is used to predict the sensor performance at different scene temperatures.

To detect small infrared targets under the condition of dense clutters, we propose a single-frame target detection algorithm based on a small bounding-box filter, which is characterized by good adaptability to the position and size of a small target. During the small target detection process, the proposed algorithm first searches for the local maximum gray pixel and then, a set of concentric bounding boxes whose center is the pixel found in the first step is constructed, and the detection thresholds of a neighboring region of this pixel are calculated based on the bounding boxes. Finally, the minimum threshold is used to detect small target pixels in the neighboring region. A fast version of the proposed algorithm is a minimum bounding-box filter, which can be implemented by dividing an image into blocks and using the mid-range and range to assess the concentration trend and dispersion of the background. Simulation and analysis results show that the proposed algorithm can achieve high detection probability and low false alarm rate when detecting small targets in the complex background; while its fast version has high computational efficiency. The proposed algorithm can be used in infrared searching and tracking systems.

Thermal effects of power cables are critical factors for determining the remaining lifetime and efficiency of power grids. Herein, we propose a fiber Bragg grating (FBG) sensor package with high-temperature sensitivity. Using polyacetal, which has a higher coefficient of thermal expansion than typical optical fibers, we constructed the proposed sensor structure. The developed sensor exhibited about 14.1 times higher temperature sensitivity than a typical bare FBG sensor. To measure the temperature variation of power cable joints caused by partial discharge, we constructed a mock system similar to a manhole, and we installed the developed sensor at a 22.9-kV crosslinked polyethylene cable joint. The experimental result reveals that even a slight temperature difference of 0.2°C can be measured using PD (300 pC at 20 kV).

We have constructed, calibrated, and tested a cryogenic low-background infrared (IR) radiometer for both spectral radiance and irradiance measurements over the 4- to 20-μm wavelength range. The primary purpose of the Missile Defense Transfer Radiometer (MDXR) is to measure absolute irradiance or radiance from cryogenic IR test chamber sources using a photoconductive Si:As blocked-impurity band (BIB) detector and a set of spectral filters. The MDXR also includes an absolute cryogenic radiometer (ACR) and a Fourier-transform spectrometer (FTS). For irradiance measurements, the ACR is used to provide the primary power scale for the BIB detector in conjunction with spectral filters, whereas the FTS/BIB configuration derives its scale from an internal blackbody source. The two measurement scales show agreement for the irradiance of highly collimated (<1  mrad) IR beams from 10^−  13 to 10^−  8  W  /  μm  /  cm² within the combined relative uncertainties of 2.6% (coverage factor k  =  1.) We have also calibrated the radiometer for radiance measurements using a large cavity fluid bath blackbody that overfills the spatial and angular extent of the radiometer entrance pupil. The radiometric calibration uncertainty analysis of the radiometer and its maintenance and stability are discussed.

A method of simultaneous and precision measurement of yaw and pitch based on digital speckle pattern interferometry is proposed. The relationship between the yaw/pitch and phase distribution is analyzed in detail, resulting in the establishment of the yaw and pitch simultaneous measurement model. A plane fitting method is proposed to separate the two angle motions from a single phase map. The proposed method enjoys the advantages of being free of cooperative target, high-resolution measurement, and compact optical setup. Experimental results show the mean absolute error of the measurement is <1  μrad.

Laser Doppler vibrometry-based voice detection systems can measure voice signals without direct contact by demodulating the Doppler signals caused by sound vibrations. However, in remote voice detection, the coherent signal is significantly affected by noise in the detection environment. This weakens the energy of the signal and reduces the signal-to-noise-ratio (SNR), resulting in the signal being incorrectly demodulated. We present a coherent signal demodulation method based on generative adversarial networks (GANs) called a phase demodulation generative adversarial network (PDGAN) that can overcome this problem. To develop the method, coherent signal and Doppler signal datasets were also established. Demodulation from a coherent signal is accomplished through unsupervised learning within the PDGAN, layer by layer, with global supervised feedback learning for fine-tuning. Experiments confirm that the PDGAN method can demodulate Doppler signals when the SNR is as low as −20  dB, with the demodulation effect being notably better than traditional phase demodulation methods. The PDGAN method is also robust against noise, so it can extend the measurement distance for laser voice detection. Generally, the method’s performance confirms the applicability of deep learning approaches for optical measurement.

We propose a clustering-analysis approach that addresses the bandpass filter (BPF) detection problem in the analysis of the peak position of the interference envelope in vertical-scanning wideband interferometry. Our approach exploits the fact that the fringe signal formed by a wideband light source is a superposition of deterministic fringe signals and noise. Consequently, two clusters can be specified in advance for use in the k-means clustering analysis. We describe our measurement procedure and its development using a two-means method-based spectrum selection scheme based on the constituent characteristics of interference fringes. The details of our approach are introduced within the framework of femtosecond optical frequency comb-based interferometry. Our experimental results demonstrate the superiority of the proposed algorithm over the conventional heuristic threshold-based BPF estimation for envelope peak determination.

High-resolution submarine seismic survey techniques have become necessary in many actual geological and geophysical investigations. A new type of mini-distributed acoustic sensing (DAS) module is developed for working at the bottom of the sea with several kilometers long single-mode fiber cable for tens of thousands of channels at the same time. Integrated designs of optics and electrics help to significantly reduce volume and power consumption. Compared with a common land-based DAS system, the size and power consumption of the mini-DAS module are significantly optimized. The size is 150  ×  300  ×  110  mm³ (width  ×  depth  ×  height), and the power consumption is down to 25 W. The spatial sampling resolution of ∼0.8  m is retained for high-resolution seismic profile in the deep sea survey. The upper limit of response frequency is set by 500 Hz for the channel sample rate of 1  kS  /  s to realize the long-term data storage. It presents a powerful signal acquisition ability with the average system noise of 4.79  ×  10^−  4  rad  /  √Hz and the minimum detectable strain is 10.4  pε  /  √Hz. The innovative mini-DAS module has high enough capabilities for real-time seismic wave signal detection in deep sea.

We present a precise passive frequency difference stabilization scheme for the Y-shaped cavity dual-frequency laser. A two-stage thermostat and a precision steady current circuit were designed to stabilize the temperature and discharge current of the laser. The laser relative frequency difference drift rate is stabilized within 0.011% at room temperature. The result shows an improvement of two orders of magnitude over the stress-induced birefringence closed-loop control method. This scheme will further improve the accuracy of acceleration and precision force measurement. Moreover, the scheme shows the great potential in industrial applications because of the low cost and good environmental adaptability. In addition to the temperature change, we found that the mode repulsion also affects the stability of the frequency difference.

We propose a differential laser Doppler velocimeter (LDV) for two-dimensional (2D) distribution measurements of the in-plane velocity component based on bias-frequency spatial encoding. Using two crossing beam arrays, the structure can effectively assign bias frequencies to the measurement points over a 2D area. As a feasibility study of the proposed method, velocity distribution measurements were conducted using a 4  ×  4-channel optical setup. The experimental results indicate that the proposed method could successfully measure the 2D distribution of the in-plane velocity component.

A color-coded computer-generated Moiré profilometry (CGMP) with real-time 3D measurement and synchronous monitoring video collection is proposed. In this method, a sinusoidal grating and the direct current (DC) component of it are encoded in the blue and red channels of a color composite grating, respectively. This color composite grating is projected onto the measured object, and a corresponding color deformed pattern is sampled by an RGB camera. A monochrome sinusoidal deformed pattern and a flat image are demodulated from the color deformed pattern by color separation. The flat image reflects the DC component of the monochrome sinusoidal deformed pattern and the monitoring image in real time. The DC component of the monochrome sinusoidal deformed pattern is efficiently figured out by multiplying the flat image with an introduced coefficient, even though the DC spectrum and the first-order spectrum of the monochrome sinusoidal deformed pattern overlap. By establishing a color-coded CGMP, the 3D shape of the measured object is reconstructed successfully. Due to its single-shot feature, the 3D information and synchronizing monitoring video are sampled in real time. Some static objects and moving objects are measured to verify the feasibility, validity, and accuracy of the proposed method.

The paper is devoted to a relatively new, promising class of lidar systems called S-lidars (the S symbol comes from Scheimpflug), which is gradually becoming a recognizable, popular laser remote sensing tool. We analyzed their potential range-domain capabilities from different angles. A generalized structure of S-lidars combining the methods and approaches used has been developed that emphasizes specific features of their design and operation and distinguishes them from classic lidars as predecessors. Based on the introduced Q-factor of range resolution quality, we proposed to describe the boundaries and variability of the operation range as the depth of field of sharply focused images, where the required spatial selectivity is still provided. It has been shown how to optimize the range-domain characteristics of S-lidars according to the criterion of maximum operation range length and how to justify and algorithmize the proper selection of instrument parameters. We have proposed a range-domain efficiency criterion of S-lidars based on comparison of any real lidar with an optimized one in terms of achieved operation range taking into account its possible narrowing from far and near border. Requirements for lidar parameters were formulated to provide a range resolution of 1 m at the far border, remote at 1 km. An algorithm for solving the synthesis problem as a sequence of selecting instrumental parameters to ensure the required characteristics has been developed. The results of the analysis and synthesis of S-lidar systems under discussion can be adapted to a wide range of specific applications, which confirms the prospects of this class of imaging-based laser diagnostic devices.

We demonstrate that the dispersion of the refractive index (RI) influences the retrieval accuracy of the particle size distribution (PSD) in the multi-wavelength angular scattering measurement. A numeric simulation was performed, which indicated that the dispersion results in a 10% to 50% error on scattering intensity measurement at certain scattering angles. In addition, although the dispersion showed little effect on the ill-posedness of the scattering kernel, the difference between the dispersion and no dispersion in the scattering kernel is so amplified in PSD inversion that accuracy decreases to such an extent that it becomes unacceptable. The accuracy of PSD retrieval, considering dispersion, was higher than that without dispersion. Finally, it was also proposed that variation of RI with incident wavelength must be considered in designing the scattering kernel to improve the retrieval accuracy in the multi-wavelength angular scattering measurement.

The bit generation algorithm of a quantum random number generator based on photon arrival time differences is improved in terms of the bit generation efficiency and bit generation rate. The method is analyzed mathematically, proving that, in theory, the improved generator still produces random numbers of high quality without the need of resorting to postprocessing algorithms. The effects of nonidealities are quantitatively analyzed to provide bounds for the maximum tolerable deviations from uniformity. Experimental investigations are conducted on a prototype, showing that the method is capable of increasing the rate of bit generation by more than 47% on the same physical architecture.

For industrial three-dimensional (3D)-measuring applications as well as for other applications such as driver assistance systems in vehicles, a high-quality reproduction in a depth map calculation is of paramount importance. Yet to date, light field cameras still exhibit severe problems in imaging homogeneous surfaces such as smooth metallic surfaces, plastics, or semiconductor materials. On those, the necessary contrast for determining the topography is not present or too weak. Here, we present an innovative approach to improve the performance of light field cameras (also called plenoptic cameras) for various applications in 3D measurement technology. More specifically, we show that this disadvantage can be avoided using structured lighting pattern and analyze various lighting patterns (with respect to geometry, size, regularity, etc.) used to improve the performance of the algorithm in greater detail. In addition, different ways of projecting the pattern have been examined with regard to their advantages and disadvantages. We perform a systematic investigation of the impact of structured light illumination with regard to different object materials, object geometries, and illumination patterns as well as illumination sources. These examinations further comprise the analysis of environmental influences and magnitude of measurement deviations in a light field camera measuring set-up. Summing up, we investigated experimentally the influence of structured illumination on the performance of 3D depth information without the need of additional phase information.

Structural–dimensional synthesis of laser variosystems (LVSs) for the formation of output Gaussian beam waist with smooth changes in its position and size according to a given law is considered. Synthesized variosystems can be used in setups and technological complexes with extended functionality: processing of materials, manufacturing of complex spatial configuration parts, moving micro-objects, etc. The method of automated dimensional synthesis of two-component LVSs on different operation principles with constraints on their parameters has been developed using the theory of laser optics. The method allows determining the best dimensional parameters of LVS, and in their absence, options of changing the initial data to obtain the solution of dimensional problem. Application of the method is demonstrated by examples of dimensional synthesis of LVSs of different types and extended functionality.

Motivated by recent developments in augmented reality display technology and notably smartglasses, we present a geometric framework to model the optical effects of deformations of planar holographic optical elements into curved surfaces. In particular, we consider deformations into shapes that do not preserve the Gaussian curvature, such as deforming planar regions into sphere segments.

This paper highlights the implementation of all-optical mode alteration using a simple micro-ring resonator (MRR). The polarization-based switching happens if a suitable input polarized light is applied as a source and pump power. The mode-conversion with respect to change in the behavior of the input and in the pump signal is used to realize all-optical OR/NOR and XOR/XNOR logic gates in both the output ports of the single MRR simultaneously. The all-optical switching behavior is also justified by its time-graph. The model is verified using the finite-difference time-domain simulation approach. Some characteristics parameters are also explained to highlight its benefits. The architecture may also be used to design new polarization-conversion centered logical and arithmetic circuits.

A dielectric elliptic cylinder is chosen as a nanostructure unit to design unusual deflecting metasurface and multifunctional metalens. Based on Pancharatnam–Berry (PB) phase principle, the abrupt phase change is investigated by adjusting the rotation angle of the nanostructure unit. The relationship between abrupt phase change and rotation angle is optimized. Using this relationship, we have designed different metasurface elements working in the visible-light range. The designed metasurfaces can arbitrarily control the phase of the light wave, and also convert the incident left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light into each other. One of the designed structures is a deflecting metasurface, which can deflect incident LCP and RCP light by the same angle while the bending directions are different. By cascading multiple gradient phase metasurfaces, an innovative type of multiplex beam splitter is constructed. The other is a metalens that can converge the incident LCP light and diverge the incident RCP light. In addition, two types of bifocal metalenses are dexterously designed using alternately arranged nanostructures, which can arbitrarily control two focal lengths of each metalens. This is promising for developing nano-integrated elements and interconnection.

Stress formation during the recording and bleaching of photopolymer holograms results in distortion of the internal diffractive structure and causes many undesirable effects. This is especially important in the case of holographic solar concentrators and similar holographic optical elements that are supposed to meet stringent specifications. We report a new digital holographic method to measure and study stress formation in photopolymers during hologram recording. A glass substrate, with one of its sides diffusely reflecting, is effectively applied to facilitate the recording of good digital holograms captured using a CMOS camera. A green-sensitive photopolymer is used for the study. Digital holograms recorded in successive time intervals of exposure were overlaid and numerically reconstructed. High-contrast stress fringes were generated, and the preliminary results are presented.

Optical designs for the next generation space science instruments call for unconventional, aspheric, and freeform (FF), prescriptions with tight tolerances. These advanced surfaces enable superior-performance, compact, and lower cost systems but are more challenging to characterize and, hence, to fabricate and integrate. A method was developed to characterize a wide range of optical surfaces, without requiring custom-made correctors, and to align them to each other for a high-performance optical system. A precision coordinate measuring machine, equipped with a non-contact, chromatic confocal probe, was used to measure numerous optics including large convex conics, high-sloped aspherics, several FF surfaces, and grazing-incidence x-ray optics. The resulting data were successfully reduced using custom-developed, advanced surface fitting analysis tool, to determine the optic’s alignment relative to the global and local coordinate systems, surface departure from design, and the as-built optical prescription. This information guided the modeling and the alignment of the corresponding as-built optical systems, including a flight system composed of a three-mirror anastigmat.

The precision of the strapdown inertial navigation system (SINS) depended on the work precision of the gyroscope. However, many errors affect the precision of SINS even if the gyroscopes have perfect principles and structures. Before the application of the inertial navigation system (INS), the inertial components must be calibrated to ensure the performance of components, detect whether the precision of the inertial component satisfied the system, and compensate for the error of INS. According to the mathematical model of fiber optic gyroscope (FOG), we propose a multi-position iterative recursive calibration algorithm to compensate for the error of scale factor and installation of FOG. To guarantee the accuracy of the SINS, the error mathematical model of the FOG is established and compensated in the system. Finally, the calibration parameters of FOG are calibrated by the multi-position iterative recursive calibration (MPIRC) and the six-position method. The calibration results show that the accuracy of components calibration parameters of the MPIRC and the six-position is similar, but the accuracy of SINS using MPIRC is higher than that using six-position.

We present a method for transferring images with high spatial resolution through highly scattering media. Rather than achieving high resolution (HR) using a larger transmission matrix (TM), which is difficult to implement in practice, we propose an image coding scheme based on compressed sensing, which is capable of recovering object information at low sampling rates, to enhance TM-based image reconstruction. The essence of the proposed scheme can be considered as an exchange of spatial and temporal resolution. By appending extra images, which are compressive encoded signals of the high-frequency information of the original object, following transmission of the low-resolution image, HR image of the original object can be reconstructed from the measured speckle signals. The feasibility of the proposed method is validated by optical experiments. The proposed method achieves a remarkable resolution improvement without relying on a complex optical system, thereby indicating promise for existing applications involving image transmission through scattering media.

Compared to the rapid development of design methods, the optimization of the GeSbSe layer design is still lagging behind. Accordingly, the layer design should be acceptable for existing methods. Optimum selection of GeSbSe layer design in connection to transmission and thickness choice is discussed. The Ge_xSb_{40  −  x}Se₆₀ (x  =  12, 25, and 30 at.%) were synthesized from elements with 5N purity (Ge, Sb, and Se) using the conventional melt-quenching method. The experimental measurements are obtained in spectral range 250 to 1700 nm using spectrophotometer UV-VIS Lambda correlated with theoretical simulation obtained by OpenFilters. OpenFilters allows the creation of a material with higher transmission and low thickness for the possible applications in infrared transmitting filters and three-level phase-change memory.

Self-luminous display devices are essential in various working environments, such as aircraft cockpits and the driver cab of vehicles and trains, where the external light environment varies drastically. Owing to the significant illumination changes, auto-adjusted display luminance based on ambient lighting is necessary for drivers to work efficiently and comfortably. This study proposes a display dimming model based on three dimensions of ergonomic testing, consisting of visual performance (VP), visual comfort (VC), and visual fatigue (VF). Five ambient illuminances, each combined with five display luminance levels, form a total of 25 conditions demonstrated in this experiment. Using a within-subject design, ten observers experienced all the combination conditions. The experiment employs the Anfimov table to test the VP, VC scale to evaluate VC, and VF scale to assess VF. Based on the experimental results, sub-models are constructed to clarify the characteristics of each dimension (VP, VC, and VF). Subsequently, the analytic hierarchy process is employed to construct an evaluation system by calculating the weight of each dimension in the total score. Finally, exponential fitting is utilized to build a wide-range display dimming model, which explicitly describes the inherent connection of complex light environment matching.

Visible light communication (VLC) has the ability to provide a high data rate up to Mbps in underwater environments for real-time communication systems. In underwater VLC (UWVLC), two major impairments are the turbulence-induced fading due to variation of salt and temperature of seawater and incremental path loss with distance due to absorption and scattering. We consider these two impairments and derive the closed form expressions for average symbol error probability (ASEP), asymptotic relative diversity order, and ergodic capacity for UWVLC dual-hop cooperative communication system. We consider multiple receiver branches with selection combining to combat the effect of fading. The impact of temperature on the fading parameters and system performance is highlighted. We conduct a comparative analysis of ASEP for four-pulse amplitude modulation and four-square quadrature amplitude modulation schemes and draw useful insights. We prove the accuracy of the derived analytical expression using Monte Carlo simulations.

We have numerically examined the advantages of thickness- and composition-grading of the electron blocking layer (EBL) in InGaN multiquantum well light-emitting diodes. We have enhanced the hole confinement inside the active region, which is critical in GaN-based devices. Low hole injection is more severe when conventional wide bandgap AlGaN EBL is inserted between the last GaN quantum barrier and the p-GaN layer. The results obtained show reduced valence band offset leading to improved hole injection and enhanced device performance.

We demonstrate a method for all-optical multi-wavelength regeneration based on four-wave mixing. Three wavelength division multiplexed signals, each having a data rate of 10 Gbps and a fixed wavelength spacing of 1 nm, are modulated with a sinusoidal signal of 25 GHz. The resultant sidebands of the modulated signal are filtered at a variable wavelength spacing and combined with a pump signal. The combined signal is passed through a highly non-linear fiber to generate multiple carriers at different wavelengths through four-wave mixing. Filtering the data modulated carriers obtained at the output of the highly non-linear fiber results in signals having lesser noise contribution. Two different optical signal-to-noise ratios (OSNRs) of 16.5 and 19 dB at the input of the regenerator are considered to evaluate the system performance. In both cases, a significant improvement in the bit error rate (BER) of the signals is observed. Simulation results show an average improvement of 2.537 and 1.335 dB in received optical power at BER of ^{10  −  9} in case of 16.5 and 19 dB OSNRs, respectively.

We present a comprehensive model to study the atmospheric channel impairments within the low-and mid-latitude regions for the geostationary satellite-to-Earth-station (GEO-ES) links in the selective terahertz (THz) bands (140, 220, 345, and 410 GHz), in terms of dispersion, turbulence loss, as well as atmospheric attenuation in different seasons. Our analysis shows that dispersion effect in the channel above 300 GHz causes serious performance degradation in the low-latitude (LL) regions compared to mid-latitude regions, resulting in a need for a higher signal-to-noise-ratio (SNR) to achieve an expected bit-error-rate of, e.g., ^{10  −  3}. On the other hand, the GEO-ES link budget requirement becomes more stagnant due to turbulence loss in the LL regions compared to mid-latitude regions. In addition, seasonal variations investigation indicates that an improvement of 2- to 5-dB SNR can be obtained in the mid-latitude regions during winter. The influence of channel impairments on 16-QAM performance in an opto-electronic THz link with slant path geometry is analyzed as an example, and our results suggest that GEO-ES links operating in the 140- and 220-GHz bands are more foreseeably realistic in the low- and mid-latitude regions, whereas the 345-GHz band also shows a good potential for the future Earth-satellite THz links with high data capacity.

Ultraviolet (UV) communication can work effectively in non-line-of-sight (NLOS) mode due to its special scattering property. An NLOS channel model suitable for UV communication network is established. The relationship between path loss and emission beam angle, emission elevation angle, and transmission distance is analyzed, and the effectiveness of the model is verified by experiment. Based on the established NLOS channel model, a diamond communication network design is proposed for the UV communication network, the effective coverage area expression of the diamond network and the maximum effective coverage rate of 44.23% are obtained, and the minimum number of nodes required for the complete seamless coverage of the communication network is analyzed. By comparing the diamond network and the square network under the certain connectivity, the effective coverage area loss rates of the diamond network and the square network are 1.504% and 21.083%, respectively.

A simple scheme of tunable parabolic pulses generation is proposed. Mathematically, a parabolic function is a quadratic function, which is equivalent to a sine-squared function when the independent variable is small enough. Since the transmissivity curve of Mach–Zehnder modulator is exactly a sine-squared function, bright parabolic pulse or dark parabolic pulse can be obtained that a linear drive signal with small voltage is used to map a center range of the peak or valley of the curve to the time domain. Starting from a sinusoidal signal, the second-order approximation triangular signal is simply obtained and used as a drive signal. By setting the bias point appropriately, parabolic pulses with the same or double frequency as the drive signals can be generated. Theoretical analysis and simulation are given. In the experiment, full-duty-cycle parabolic pulses at 2, 3, and 4 GHz and corresponding frequency-doubled pulses are obtained, which are consistent with the theoretical expectations.

In coherent optical time-domain reflectometry, external modulation is used to maintain the coherence of laser probe pulses launched into optical fibers. However, the residual continuous wave (CW) component produced by modulation may considerably degrade the system sensitivity. The backscattered signal from the pulse must be dominant compared to the CW signal. We discuss the effects of the finite extinction ratio (ER) on the instrument’s sensing range. A model analyzing the impact of the CW component on the backscattered signal as a function of the ER, fiber length, and pulse widths is proposed. It is also shown that acousto-optic modulation is more suitable than electro-optic modulation in optical fiber for longer distances. The results are confirmed experimentally in a 31–km-long fiber. A 1-Hz vibration was applied at 25.5 km, and the resulting signal-to-noise ratio of ∼13  dB was measured using 75-ns pulses. This result is in agreement with the performance predicted by the model.

We report a large-core radial-firing optical fiber tip comprised of conically shaped air-pocket, fabricated by deforming a hollow optical fiber fusion-spliced with the large-core optical fiber (LCF) (core/cladding diameters  =  200  /  220  μm) using the arc-discharge method. The effect of the intaglio air-pocket angle and the numerical aperture (NA) of the LCF on the radial-firing characteristics of the fiber tip was investigated. The design of air-pocket of the fiber tip was optimized for an effective radial-firing by simulating the beam profile of the radial-firing optical fiber with the ray-tracing method. The 45 deg of the conical angle with the low NA (0.12) of the LCF has shown the maximum radial-firing angle of 81 deg by the simulation. The fabricated LCF tip with the intaglio conical air-pocket with the 45 deg angle has shown the radial-firing angle up to ±78  deg, which was in good agreement with the simulation results. The present LCF radial-firing tip can be an effective element of medical devices for treatment of tubular shape tissues with the high-power transmission and the ease of operation in vivo.

A broadband fiber-optic parametric amplifier (FOPA) based on near-zero dispersion profile with single zero dispersion wavelength (ZDW) by selective liquid infiltration technique is numerically investigated. Here, the gain and bandwidth for an FOPA incorporating “dual pumping” scheme are thoroughly investigated by varying the fiber length, operating wavelength (central wavelength), and pump power. The structure is engineered in such a way that the dispersion profile is having near-zero dispersion value (negative β₂) for a wide wavelength range and small positive β₄ around the pump wavelength; thereby, the phase mismatch term remains within the favorable value for a wide wavelength leading to wider FOPA sources. The structure also supports reasonable high nonlinearity and low propagation loss. Our numerical investigation results in an FOPA with optimized bandwidth of around 180 nm close to communication wavelength. Finally, a detailed comparative study of the single pump scheme and dual pump scheme for FOPA/amplifier gain and spectrum bandwidth has been performed for identical fiber parameters. It has been observed that dual pump scheme provides smoother/flatter spectrum (lesser gain fluctuation) around the center of the optimized profile with slightly lesser bandwidth compared to single pump operation. Additionally, it was observed that the dual pump FOPA supports higher pump power thereby enabling higher FOPA unaltered peak gain (3 dB) throughout the bandwidth (the peak and dip of the gain spectrum difference remaining within 3 dB) than that of single pump scheme.

Massive data traffic demand for satellite networks has been increased with the emergence of global broadband services. Due to limited onboard processing capacity and imbalanced distribution of global traffic, severe link congestion may occur in hot spots, deteriorating the performance of the whole network. A link cost modification applied in load balancing algorithm based on artificial bee colony is proposed for routing and wavelength assignment problem to distribute traffic in advance. Moreover, congestion avoidance based on wavelength utilization is designed for unexpected congestion to minimize the adverse impact of congestion on network performance. Experiment results show that the method we proposed realizes high performance in communication success rate and average transmission delay and also achieves the maximum utilization of wavelength utilization under the condition of low blocking rate and the equalization of traffic load.

Thulium fiber laser (TFL) lithotripsy has recently been introduced for clinical use. Previous TFL laboratory studies demonstrated high-power delivery through small (50- to 150-μm core) optical fibers. This preliminary study simulates forces on fibers during insertion into a flexible ureteroscope and determines mechanical feasibility of small fibers for a clinical setting. Simulations were conducted with commercially available fiber (core/cladding) sizes of 50/70, 72/108, 100/140, 150/165, 150/180, and 200  /  240  μm. Solidworks software integrating Euler’s buckling equation was used to calculate fiber buckling thresholds as a function of typical manual forces (0.15 to 2.0 N) applied near the proximal end of a ureteroscope. Forces on fibers were modeled assuming support from saline flow and resistance by the working channel wall. Simulation results were categorized based on force values previously reported in the literature, with smaller forces (<0.4 to 0.8 N) buckling fibers, mid-range forces (0.8 to 1.6 N) optimal for fiber manipulation, and higher forces (>1.6 to 2.0 N) at risk of damaging the ureteroscope working channel. Fiber sizes were simulated with two different types of holdings on each end to find a range of possible values that most closely simulate clinical behavior. Simulation results were confirmed using a force meter and a benchtop experimental setup. Numerical simulations predicted that optical fibers for TFL lithotripsy should be equal to or larger than 150  /  190  μm (core/cladding), for effective manual manipulation within flexible ureteroscopes.

The performance of the underwater wireless optical communication (UWOC) system is severely affected by the presence of air bubbles in the water. Therefore, an experimental setup is designed to investigate the effect of different sized air bubbles on the laser beam propagating through the UWOC system. A wide range of air bubbles has been generated in the water tank by varying the diameter of the holes in a copper tube and by varying the air flow rates. When an optical beam propagates through the underwater environment in the presence of air bubbles, the received optical beam undergoes severe intensity fluctuations. The distribution of these intensity fluctuations conveys very useful information to predict the behavior of the underwater channel. The distributions of these intensity fluctuations have been modeled using a Gaussian mixture model (GMM), which is the sum of Gaussian functions. The parameters of the proposed GMM model are estimated by expectation maximization algorithm to obtain maximum likelihood estimation. The goodness of fit is also performed by considering a confidence interval of (95%) to estimate the feasibility of the proposed GMM model, which provides excellent results. The proposed GMM model excellently fits the experimental data for all the considered cases. In addition, based on this proposed model and experimental observations, the performance of the UWOC system is evaluated in terms of signal-to-noise ratio, outage probability, bit error rate, maximum Q-factor, etc. The results show an exact match between the experimental and proposed theoretical results that are modeled using GMM model and hence signifies the precision of the proposed model.

An analysis of optical scintillation and fade on long slant-path atmospheric channels is presented via a direct comparison between wave-optics-based numerical simulations and experimental flight data from a ground-to-aircraft optical communication link. In addition to physically modeling the propagation through slant-path atmospheric turbulence, the numerical simulations include simultaneously the effects of mechanical pointing jitter, aperture-averaging, and first-order scattering/absorption models. The power spectral density, fade probability, and mean fade time of the simulated power fluctuations are studied and validated against measurements taken at slant-path distances ranging from 60 to 113 km and aircraft speeds up to 70  m  /  s.

We demonstrate a high data rate and low-cost OM4 multimode fiber (MMF) and free-space optics (FSO)-based self-restorable 2  ×  100  Gbps per wavelength intra data center interconnect between two pairs of servers. This transmission system is based on two vertical cavity surface-emitting lasers that are directly modulated with PAM-4 data at the rate of 100 Gbps and multiplexed using mode division multiplexing of two linearly polarized modes. In case a fault occurs in the MMF-based primary path, the traffic is shifted to FSO-based protection path through a specially designed triggering and switching mechanism. Offline digital signal processing is used at the receiver to process the received PAM-4 signal for compensating the effects of channel impairments. The performance of the proposed architecture is evaluated at link lengths of 35 and 70 m under normal and faulty scenarios. The effect of varying the refractive index structure parameter of the log-normal channel model and the effect of mode coupling between linearly polarized (LP) modes on the bit-error-rate performance is also analyzed under different conditions. We observe that the performance of the FSO-based protection path is better than the OM4-based primary path at a forward error correction limit of 3.8  ×  ^{10  −  3} bit-error-rate.

We report the transmission of signal light and the noise accumulation in the fiber Raman amplifier (FRA) under four-wavelength time-division multiplexing (TDM) pumping. A quality factor that is used to describe the performance of the Raman fiber amplifier system in quantitative analysis is proposed. The results indicate that the power variation of signal light has a certain periodicity that is related to the period of pumping with the TDM pumping. The shorter the pumping period is, the weaker the signal fluctuation is. Through the analysis of noise, it is found that amplified spontaneous emission is the main source of the noise of FRA with TDM-pumping. By comparing the TDM pump scheme with the continuous multiwavelength pump scheme, it is found that the gain of the TDM pump scheme is higher than that of the continuous multiwavelength pump scheme.

The spread of antibiotic-resistant pathogens poses a global threat to public health, food security, and sustainable development. The design of new scientific tools based on nanoparticle (NP) surface plasmon resonance detection is relevant for the study of antibiotic resistance mechanisms. Specific aim of the research was to obtain stable conjugates of silver NPs (AgNps) with antibiotic for the study of antibiotic-resistance mechanisms. NPs with similar size and shape but with antibiotics as surface stabilizing agents were synthesized. We obtained stable conjugates of AgNPs with teicoplanin, vancomycin, chloramphenicol, cefotaxime, cefuroxime, ceftriaxone, ampicillin, and novobiocin. NPs with penicillin, cefazolin, amikacin, apramycin, kanamycin, gentamicin, and erythromycin were not stable and aggregated immediately after synthesis. The maximums of the surface plasmon resonance absorbance of all conjugates were at a wavelength of 400  ±  7  nm. Transmission electron microscopy results showed disperse NPs spherical in shapes with size from 2 to 20 nm with average diameters of 10 to 11 nm for all obtaining conjugates. Dialysis was the most appropriate procedure for AgNPs-antibiotic conjugates purification, whereas ultracentrifugation at 100,000 g lead to aggregations of NPs and loss of optical nano-properties. The optimal temperature for conjugates storage is 4°C and pH 6 to 8. AgNPs-antibiotic conjugates exhibited stability under different conditions (temperature, pH, and biological mediums). These features and strong optical and antibacterial properties make AgNPs-antibiotic conjugates as attractive tools for the study of antibiotic resistance mechanisms.

The effects of solar spectral irradiance on the performance of laser spot tracker are analyzed. When the entrance pupil diameter of the receiver increases, the field of view decreases, but the detection range increases. If the angle between the receiver and transmitter and target increases from 0 deg to 60 deg, the detection range decreases by approximately 23%. Due to solar noise’s effects caused by solar spectral irradiance, the detection range decreases by approximately 33% during the day, more than during the night, and the laser spot tracker’s altitude decreases by approximately 13% at 2000 m, more than at 0 m. When the solar intensity’s variation increases from 1% to 5%, the detection range decreases by 54%.

We propose a method that uses the back propagation (BP) neural network algorithm to optimize the design of the multipump Raman fiber amplifier. We determine the optimal training model by examining the number of hidden layers in the multilayer BP neural network and the number of neural nodes contained in it. The model more accurately reflects the mapping relationship between the wavelength and output of the pump light and the Raman net gain distribution, instead of the traditional method of solving the Raman-coupled wave equation. The experimental results show that, using the trained BP neural network model to train new validation datasets, the studied Raman amplifier achieves the desired performance, and the maximum error between the target value and the predicted value does not exceed 0.3 dB. Compared with previous studies, this design scheme improves the accuracy of model calculation and the optimization efficiency of the Raman amplifier.

This erratum corrects an error in Eq. 5.

This erratum corrects a typographical error in a value, which affected some results in the paper.